1. Field of the Invention
This invention relates to charged particle energy analyzers. In particular, the invention relates to charged particle energy analyzers for the analysis of the energy of charged particles, for example electrons, which have been emitted from the surface of a sample by the bombardment of the sample with primary radiation, such as soft X-rays, ultra-violet photons, or electrons.
As the energy of the emitted charged particles will be characteristic of the chemical composition of the part of the sample emitting the charged particles, the energy analysis thus provides information relating to the chemical nature of the surface of the sample.
2. Description of the Prior Art
It is advantageous to be able to obtain information relating to the distribution of particular chemical species over the surface of a sample. A number of charged particle energy analyzers have been developed which perform this imaging function. One such example is described by G. Beamson, H. Q. Porter, and D. W. Turner, in J. Phys. E: Sci. Instrum., 13. pages 64-66, 1980. In the instrument described, the sample is placed in the strongest region of a diverging magnetic field and illuminated with, for example, ultra-violet light or soft X-rays. The resultant photo-electrons spiral about the magnetic flux lines. As the magnetic flux lines diverge, so the mean photo-electron trajectories diverge, such that the photo-electrons can be caused to bombard a two dimensional detector to thereby produce a magnified image of the specimen surface.
In order to perform an energy analysis of the photo-electrons in such an apparatus, if the photo-electrons are still confined within the magnetic field, then an imaging band pass filter may be used, as described by D. W. Turner et al, in Rev. Sci. Instrum. 57(8), 1986. Alternatively, if the photo-electrons are projected into a region of low, or zero, magnetic field, an imaging energy analyzer, as described by R. Castaing and L. Henry in Comptes Rendus (Academy of Science, Paris) 255B, page 76 onwards, 1962, may be utilized. Further magnification of the energy resolved image can then be performed if required, and the final image projected on to a phosphor screen or other suitable device.
In European Patent Application No. 0246841, there is described an electron spectrometer effective to produce an image of a photo-electron emitting surface, energy analysis of the photo-electrons being performed electrostatically. The spectrometer described uses a conventional hemispherical electrostatic deflection analyzer in combination with an electrostatic lens arrangement. The electrostatic lens arrangement performs a transformation of the spatial information in an image of the photo-emitting surface into corresponding angular information, before the photo-electrons enter the space between the two hemispheres constituting the analyzer, on an equatorial plane of the hemispheres. The transformation is achieved by arranging for an electron lens to be positioned between a magnified image of the photo-emitting surface and the analyzer, with the magnified image at one focal plane of the lens, and the entrance plane of the analyzer at the other focal plane of the lens. Thus there is a direct relationship between the angle that the photo-electron trajectory makes with the electron-optical axis of the analyzer at the entrance to the hemispheres, and the photo-electron's position within the magnified image. An image which is spatially dispersed in energy is produced at the exit plane of the analyzer, the image preserving the angular information within the input image. An aperture at the exit plane can then be used to select a narrow band of energies from the beam of photo-electrons. A further electron lens is positioned such that the exit of the analyzer is at the object side focal plane of the lens. This lens will thus reconstruct the magnified image of the sample surface at the image side focal plane of the lens, which reconstructed image may be viewed directly on a phosphor screen, or other suitable device. It will be appreciated that such an apparatus necessitates the conversion between spatial and angular information, thus requiring careful alignment of the electrostatic lens arrangement.
H. Z. Sar-El in an article in Nuclear Instruments and Methods 42, pages 71-76, 1966 describes a charged particle energy analyzer including an electrostatic spherical mirror analyzer in which a baffle is incorporated between the spheres, the baffle being effective to improve the resolution of the analyzer. This analyzer is, however, only capable of imaging a point or small source.
H. Diamon, in an article in Rev. Sci. Instrum. 59(4), 1988, describes an instrument in which an aperture is placed at the image point on the equatorial plane of a pair of hemispheres constituting a hemispherical mirror electrostatic analyzer. Photo-electrons, from a point source located on the equatorial plane of the hemispheres, enter and leave the field between the hemispheres via gridded portions of the inner hemisphere. This enables a very large angular range, up to 2 steradians of photo-electrons emitted by a sample, to be analyzed simultaneously. Photo-electrons with an energy different to the pass energy of the analyzer are focused to a point before, or after, the aperture, and so are less likely to be transmitted through the aperture. Such an apparatus, however, does not perform simultaneous energy analysis of a large number of points contained within a two-dimensional surface.
U.K. Patent Application GB-2221082A describes a charged particle energy analyzer including an electrostatic spherical mirror analyzer that may be used for forming an image of secondary electrons emitted from a surface. The analyzer described does not, however, have the property that the energies of the electrons forming the image are independent of their position in the image. Thus, the analyzer is not able to achieve the high energy resolution necessary to unambiguously determine the distribution and chemical state of elements on a surface.